1. Field of the Invention
This invention relates to minimum shift keying ((MSK) transmitters.
2. Description of the Prior Art
Digital communication systems frequently have two design criteria which are of importance: the use of minimal r.f. spectrum and the ability to be transmitted through transmitters having non-linear amplitude transfer functions. The former stems from an ever increasing use of the r.f. spectrum. The latter stems from the observation that most medium and high output power transmitters perform with higher efficiency when they operate at or near saturation of their input vs. output amplitude transfer function.
Frequency shift keying (FSK), a technique of communicating digital information using discrete frequencies to represent specified symbols, has properties desirable in satisfying both of the above criteria. Binary FSK, for example, transmits a first (mark) frequency to represent a binary one and a second (space) frequency to represent a binary zero. The amplitude of the carrier, in the ideal case, is invariant and thus can pass through non-linear amplifiers with minimal degradation of signal quality. Further, proper choice of the difference between the mark and space frequencies and the time of switching between these frequencies can provide strict control and minimization of the radiated spectrum thereby minimizing the required channel bandwidth and the spurious emission of power outside the channel bandwidth. This reduces interference with other channels or services.
Minimum shift keying (MSK) is the special case of FSK wherein the bandwidth of the system is minimized (i.e. the difference between the respective mark and space frequencies is at the minimum value) also preserving an antipodal relationship between the mark and space symbols thereby making available high efficiency detection techniques. Continuous phase is maintained in the MSK signal at the transitions between the mark and space frequency signals to decrease spurious emissions of signals outside of the bandwidth, a phenomenon sometimes called "spectrum splatter".
Such conditions are met advantageously by maintaining phase-lock between the mark and space frequency signals and maintaining the separation between the mark frequency (f.sub.m) and the space frequency (f.sub.s) precisely one-half of the bit rate (R) of the binary information to be communicated, that is, EQU f.sub.m - f.sub.s = R/2 (1)
It is assumed, for the purposes of discussion, that f.sub.m is greater than f.sub.s. Such a case, however, is not a requirement of or a limitation on the system.
In order to control further the radiated spectrum for an MSK signal, the MSK signal has conventionally been band limited by a series of bandpass filters. However, the design requirements of such bandpass filters are quite complex and severe. For instance, an MSK signal is conventionally formed by generating mark frequency and space frequency signals, and producing therefrom what may be termed an MSK-intermediate frequency (IF) signal comprising a sequence of bits of the mark and space frequency signals in accordance with the binary data to be communicated. The MSK-IF signal is subsequently translated by an RF local oscillator frequency signal to the output RF frequencies. To reduce spectrum splatter, the MSK-IF signal is passed through a bandpass filter prior to being translated by the local oscillator frequency signal thereby attentuating sidebands.
In practice, the use of complex bandpass filters in many applications is disadvantageous, in addition to the difficulty in design, in that the expense, required volume and weight of such filters increases as a function of its complexity.
The present invention reduces the severity of the requirements on the bandpass filters by utilizing, for the production of the keying signal, an analog switch which, in itself, acts as the equivalent of a bandpass filter. Moreover the present invention further reduces the severity of the design requirements on the bandpass filters by making possible the use of this bandpass filtering at IF rather than RF frequencies. Thus, less stringent and complex filters may be utilized in the transmitter and the expense, size and weight of the transmitter is thereby reduced.